User's Manual

THE 80286 INSTRUCTION SET
CALL GATE TO MORE PRIVILEGE:
Get new SS selector for new privilege level from TSS
Check
selector and descriptor for new SS:
Selector must not be null else #TS(O)
Selector
index must be within its descriptor table limits else #TS (SS selector)
Selector's RPL must equal DPL of code segment else #TS (SS selector)
Stack segment DPL must equal DPL of code segment else #TS (SS selector)
Descriptor must indicate writable data segment
else #TS (SS selector)
Segment PRESENT else #SS (SS selector)
New stack must have room for parameters
plus 8 bytes else #SS(O)
IP
must be in code segment limit else #GP(O)
Load new
SS:SP value from TSS
Load new CS:IP value from gate
Load
CS descriptor
Load
SS descriptor
Push
long pOinter of old stack onto new stack
Get word count from
call gate, mask to 5 bits
Copy parameters from old stack onto new stack
Push return address onto new stack
Set CPL to stack segment DPL
Set RPL of CS to CPL
Else
CALL
GATE TO SAME PRIVILEGE:
Stack
must have room for 4-byte return address else #SS(O)
IP
must be in code segment limit else #GP(O)
Load
CS:IP from gate
Push return address onto stack
Load code segment descriptor into
CS-cache
Set
RPL of CS to CPL
CALL TASK
GATE:
Task gate DPL must be
~ CPL else #GP (gate selector)
Task gate DPL must be
~ RPL else #GP (gate selector)
Task Gate must be
PRESENT else #NP (gate selector)
Examine selector to
TSS, given in Task Gate descriptor:
Must specify
global In the local/global bit else #GP (TSS selector)
Index must be within GOT limits else #GP (TSS selector)
TSS descriptor AR byte must specify available TSS (bottom bits 00001) else #GP (TSS
selector)
Task
State Segment mUllt be PRESENT else #NP (TSS selector)
SWITCH_TASKS with nesting to TSS
IP must be In code segment limit else #GP(O)
TASK STATE SEGMENT:
TSS DPL must be ~ CPL else #GP (TSS selector)
TSS DPL must be ~ RPL else #GP (TSS selector)
TSS descriptor AR byte must specify available TSS else #GP (TSS selector)
Task
State Segment must be PRESENT else #NP (TSS selector)
SWITCH3ASKS with nesting to TSS
IP must be in code segment limit else #GP(O)
ELSE
#GP (code segment selector)
PROTECTED MODE EXCEPTIONS
FAR calls: #GP, #NP, #SS, and #TS, as indicated in the list above.
NEAR direct calls: #GP(O) if procedure location is beyond the code segment limits.
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